Effect of winding wire insulation on the reliability of squirrel cage submersible induction motor for inverter duty operation--a case study.

Subramanian, R. ; Sivanandam, S.N. ; Rani, C. Vimala 等

Introduction

It is well known fact that Low Tension AC squirrel cage induction motors form the backbone of all the industrial drive systems. The squirrel cage motors are rugged in construction and they give good performance throughout their life with minimum maintenance. For a long time their only disadvantage was in the area of variable speed applications. In lieu of their hard or stiff characteristic, these motors were not amenable to speed control. However, with the advent of AC Variable Speed Drives, the squirrel cage motors have become hot favorites in the field of variable speed applications competing with the DC motors. A speed holding accuracy of [+ or -]0.01 % is nowadays possible with AC squirrel cage motors with closed loop control.

At the same time there are some concerns with regard to the proper selection of the squirrel cage motors so that the motors fully withstand or match the unusual demands made on them from the AC drive side.

The motors can no longer operate with perfect sine-wave power and at fixed speeds. With IGBT Drive controllers, the motors are exposed to distorted supply waveforms with high frequency components and operation in a wide range of speeds. [1]-[7]

This paper brings certain analysis on the reliability of the given submersible induction motor with reference to the insulation covering on the winding wire.

Variable speed drives

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable--speed drive. These drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, micro drives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.

Variable Frequency drives (VFDs) are used in the industry and utility services for three types of application namely Energy Savings, Smooth and soft starting of large motors and Controlling drive speed for process control. Fig. 1

The VFDs are advantages in the sense that, it increases motor life due to the lower thermal and mechanical stresses. (Absence of starting inrush currents and reduced speed operation) and hence no limitation on the number of starts.

[FIGURE 1 OMITTED]

The certain drive features that affect the motor life and its performance are:

The harmonics and high voltage due to voltage doubling effect present in the inverter output voltage causing stress to the motor insulation and causing overheating of the windings (subsequently reducing the life of the motor insulation).

The presence of Common mode voltage and capacitive couplings, which become active due to the high frequencies present in the inverter power supply, causing break down of bearing insulation, flow of bearing current and damage to the bearing. Squirrel cage Submersible Induction Motor

[FIGURE 2 OMITTED]

As explained in the introduction the Squirrel cage induction motors are the most rugged and found suitable for most of the applications. One such application is bore well and open well water pumping applications. The motors used in these applications are named as bore--well submersible motors and open-well submersible motors normally in dry and wet type construction. In the recent past the motors along with VFD for head (pressure) boosting or variable head applications. The name wet type is due to the coolant used inside the motor chamber for not only dissipate the heat from the winding and other parts but also act as a lubrication agent for the bearings used irrespective of the type of the bearing whether journal or anti friction bearing.

The copper is the only conducting material used as a conductor for submersible motors. But depending on the type of motor the insulation covering on the bare copper varied. For water cooled wet type motors Polyvinylchloride (PVC) insulated or Polyester wrapped copper winding wires are used where as for oil cooled wet type motors, Modified Polyester coated and other polymer family coated winding wires are used. In general the motors are meant for continuous duty (S1 Duty) rating and the reliability of the system relies on the insulating materials used in these types of motors. [10]

Developments in the materials and varnishes used in motor insulation systems have improved the thermal, mechanical and dielectric characteristics considerably.

However,

There are three suggested possibilities for insulation damage:

1. Breakdown between coil and stator core: This is normally not a problem as slot liners are used will counter the effect.

2. Phase to phase failure--in the slots or end-windings: as the motors use interphase barriers these failures may not cause much damage.

3. Inter-turn failure between adjacent conductors in the stator winding: the most probable cause of failure due to the non-uniform distribution of voltage along the stator windings, associated with the short rise times of the incident voltage pulses.

Depending upon the homogeneity of the stator winding materials and impregnation, there may be voids in the impregnating resin. Figure 2 illustrates the details of voids that may present in the winding. It is in such voids that the failure mechanism in the inter-turn insulation occurs.

The failure mechanism is a complex phenomenon called partial discharge (PD). PD is a low energy discharge that occurs when both the following conditions apply:

The peak value of the applied voltage is lower than the actual breakdown voltage of the insulation system Motor Insulation Voltage Stresses under IGBT Inverter Operation. The local electric field intensity that is created in a void or cavity is sufficient to exceed the breakdown strength in air (Partial Discharge Inception Voltage). When subject to continuous partial discharges, the insulation system progressively degrades, ageing the insulation material

The ageing process results from an erosion of the insulation material, reducing its thickness at the discharge sites until its breakdown voltage capability is reduced to below the level of the applied voltage peak, at this stage insulation breakdown occurs.

This study is made, with modified polyester coated and dual coated winding wires (Table I), to analyze such occurrences in the motor when it operates with VFDs.

Case study

A submersible induction motor along with a VFD of the following
specifications is taken for study.

Motor
Power rating               : 9.3 kW
Supply                     : AC, 3 phase, 415V, 50Hz base freq and 5
                             to 150 Hz operation.
Duty                       : S1
Encl.and type              : IP 65 water cooled and wet type
Coolant                    : oil
Poles                      : 2
Size                       : 150mm
IS Ref.                    : IS 9283/ NEMA MG1 30/ MG1 31
VFD:
Input Supply               : AC 3 phase 415V 50 Hz
Output Supply              : AC 3 Phase 0 to 415V, 5 to 200 Hz
Encl                       : IP 54
Switching device           : IGBT
Mounting                   : Surface Mount
Design Data:
Motor Rating               : 9.3 kW
Type of Installation       : Submersible
Type of rotor              : Squirrel cage
Lamination                 : CRNO SAIL M43
Stamping                   : 0.5mm thick
Stator dia. & rotor dia.   : 142 X 75 mm
No of stator slots         : 24
No of Rotor slots          : 18
Type of Stator Winding     : Single layer Concentric
Type of rotor              : Sq. cage copper conductor
Type of bearing            : Deep groove antifriction ball Bearing
Type of cooling            : Oil
Type of mounting           : Vertical flange mount

This particular case study was conducted to analyze the suitability of the motor insulation to withstand the harmonics including spikes and over-voltages caused by voltage doubling effect.

Over-voltage: This is caused by the high rate of voltage rise (dv/dt) exhibited by the Insulated Gate Bipolar Transistor (IGBT) inverters and the long length motor cable encountered both in new as well as in retrofit installations.

Over-voltage at Motor terminals [1]-[4]: Before the advent of IGBTs, thyristors and GTOs were used in the inverter section. Over-voltage problems were also present then. But due to comparatively longer rise times, the effect of over-voltage was not so critical with those devices. In Pulse Width Modulated (PWM), Variable Speed Drives (VSD) with IGBT inverters, thousands of pulses are applied per second to the motor. These pulses could damage the motor insulation, if left unchecked. These are named as voltage spikes are on top of the D.C. pulses traveling out to the motor. Voltage rise time determines the voltage amplitude at the motor terminal as well as the voltage distribution in the individual coils. Long length motor cables from VSD behave like transmission lines and cause voltage reflection at the receiving end, namely, at the motor terminals as well as at the inverter terminals.

Reflected Wave: Cables connecting VSD's to motors also have impedance and if this impedance does not match the motor impedance then the some of the energy in the high frequency pulses can actually be reflected back off the motor's impedance. This is known as (voltage) reflection. If conditions are right a doubling of voltage can occur when an incoming pulse adds to the reflected pulse producing a doubling effect. A reflected voltage wave travels back along the cable toward the VSD. Here it could cause damage to the electronic components inside the VSD. In addition reflected voltage can also be harmful to motors and cables. Again the techniques mentioned above will eliminate voltage reflections. Cables are also subjected to these over voltages but no cable failures have been reported. This is due to the fact cables have an insulation thickness that is based, in part, on mechanical requirements.

Reflected wave at the motor terminals is computed as follows:

Voltage at motor terminals = (1+[tau]) * V inverter end (1)

Where [tau] is called the Reflection Coefficient and defined as [tau]

= ([Z.sub.Load] - [Z.sub.0]) / ([Z.sub.Load] + [Z.sub.0]) (2)

Where [Z.sub.Load] = Motor surge impedance to traveling waves with fast rising time and

[Z.sub.0] = Cable surge impedance to the traveling wave.

The cable impedance presents definite measurable surge impedance [Z.sub.0] to the traveling wave and is defined as the square root of inductance per unit length divided by capacitance per unit length. ([Z.sub.0] = [square root ot (L/C))] (3)

Even though cable's L and C parameters vary with wire gauge and cable construction, it is found that [Z.sub.0] relatively remains constant in the range of 80 to 180 [OMEGA] for 3 wires in a conduit, 4 wire tray cable and armored cables in size from #18 AWG to 500 MCM.

[Z.sub.Load] is the motor surge impedance to traveling waves with fast rising time. Experimental results show that motors below 5 HP have a surge impedance of 2000 to 5000[OMEGA], a medium range motor of rating say 125 HP has a [Z.sub.Load] of around 800 [OMEGA] and a large capacity motor of rating of 500 Hp has a [Z.sub.Load] of around 400 [OMEGA].

Whenever the cable surge impedance does not match the surge impedance of the motor, a reflected wave will occur at the motor terminals. Substitution of [Z.sub.Load] and [Z.sub.0] in the equation for ??results in the following:

[tau] = App. 0.95 for motors below 5 HP [tau] = App. 0.82 for motors in the range of 125 HP [tau] = App. 0.6 for motors in the range of 500 HP.

Substituting the value of ??in Equation-1, the following over-voltage relations at the motor terminals are obtained:

1.95 pu for motors less than 5 HP 1.82 pu for motors in the range of 125 HP 1.6 pu for motors in the range of 500 HP.

It may be noted that in the case of higher HP drive motors, use of parallel cables will reduce the [Z.sub.0] value, which in turn will increase the value of [tau]. In such a case, the over-voltage at the motor terminals would be in the region of 1.95 pu.

For the motor power rating of 9.3 kW, the value taken as 1.90 and hence the terminal voltage is found to be 809 volts. Although this voltage is within the limit of the value mentioned in the NEMA standard, this over voltage may cause damage to the insulation.

This over-voltage problem was mitigated [7] by the following methods: Use of VSD with long rise time: The VFD selected for long rise time to the value of 0.8 microseconds.

Improved insulation for the stator windings: Two cases were taken: one with modified polyester and the other with dual coated.

Restricting the motor cable length: In submersible motor- pump installations the cable can not be restricted to a confined length as the cable has to run to a depth at which the pumping set is installed.

Use of Filters: Filters as recommended by the drive manufacturer were used as the length cable used is 120m and the type of the cable is of Cross Linked Polyethylene (XLPE) insulated and PVC sheathed. Use of metal shielding or sheathing is to be considered in future scope of study.

An abstract from the NEMA standard pertaining to voltage stress is given in the tables below as guidelines for the inverter grade Motors:

NEMA Standard MG1-1993--Section IV--Part 30 [8]--General Purpose Motors used with Variable Voltage or variable frequency or both:

Voltage stress:

Motors with Base rating voltage [greater than or equal to]600V

Vpeak [less than or equal to] 1 kV Rise time [less than or equal to]2 [micro]s NEMA Standard MG1-1993--Section IV--Part 31 [9] Definite Purpose Inverter

Fed Motors Voltage spikes: Motors with Base rating voltage [less than or equal to]600V Vpeak [less than or equal to]1600 V Rise time [less than or equal to]0.1 [micro]s Vpeak [less than or equal to]2.5 pu

Results and discussions

The motor wound with modified polyester was put under endurance test of 100 hours at various input frequency by using VFD and found that the motor is not working due to ground fault after a 6 hours of continuous run at a freq of 140 Hz. The motor was dismantled and the windings were carefully taken outside for examination. It is observed that, some of the first few turns got broken at overhang portion. Fig 3 shows photo plates of the same.

[FIGURE 3 OMITTED]

The same motor wound with dual coated winding wires was made to run under the same conditions for endurance test. The motor was tested up to a freq of 150 Hz of intended use and after run for 100 hours it was found intact without any abnormality. The Insulation between the phase to phase and phase to ground found to be at 50M? without any deformation in the insulation level. The dielectric test details are given in table II.

Conclusion

From the analysis, the various attributes of an inverter grade motor have been brought out and it is observed that the dual coat coated winding wires found more suitable for inverter duty operations considering the voltage stress and spikes. It is noted that the increase in voltage rise time also influences the voltage peaks.

It is also noted that use of proper cables also plays a considerable role in reflection which can be taken as the extension of this study.

Acknowledgement

The authors wish to thank M/s PSG Industrial Institute, Coimbatore and M/s Small Industries Testing and Research Centre, Coimbatore for sponsoring the research and development work by giving their support to carry out manufacturing and testing work at their premises.

References

[1] Bell,S.; Sung J, "Will your motor insulation survive a new adjustable-frequency drive?," IEEE Transactions on Industry Applications, Volume 33, Issue 5, pp. 1307--1311, Sep/Oct 1997.

[2] Manz, L, "Motor insulation system quality in the presence of IGBT drives", Pulp and Paper Industry Technical Conference, Conference Record of 1996 Annual Volume Issue:10-14, pp.70-76, Jun 1996

[3] Paul T. Finlayson, "Industry Applications Magazine", IEEE Volume 4, Issue 1, pp. 46-52, Jan/Feb1998.

[4] Von Joanne, A.; Enjeti, P.; Gray, W., "Application

[5] Issues for PWM adjustable speed AC motor drives. Industry Applications Magazine, IEEE, Volume 2, Issue 5, Page(s):10-18 Sep/Oct 1996.

[6] Saunders, L.A. Skibinski, G.L. Evon, S.T. Kempkes, D.L. Allen-Bradley Co., Brown Deer, WI "Riding the reflected wave-IGBT drive technology demands new motor and cable considerations," Petroleum and Chemical Industry Conference, pp: 75-84,23-25,Sep1996.

[7] C.J.Melhorn, Le Tang, "Transient Effects of PWM Drives on Induction Motors, IEEE Transactions on Industry Applications", Volume 33, Issue 4, pp.10651072, July/August 1997.

[8] Bonnet, A.H. Available insulation systems for PWM inverter-fed motors IEEE Industry Applications Magazine, Volume 4, Issue 1, Page(s):14-26Jan/Feb1998.

[9] NEMA Standard MG1-1993-Section IV-Part 30-Application Considerations for constant speed motors used on a sinusoidal bus with harmonic content and General Purpose Motors used with Variable Voltage or variable frequency or both.

[10] NEMA Standard MG1-1993-Section IV-Performance Standards applying to all machines: Part 31-Definite Purpose Inverter Fed Motors.

[11] Indian Standard Specification IS : 9283 Motors for Submersible Pump sets--Specification (First Revision).

R. Subramanian (1), S.N. Sivanandam (2) and C. Vimala Rani (3)

(1) Research Scholar, SNS College of Technology, Coimbatore-641 035, Tamilnadu, India. E-mail: rama.smani@gmail.com

(2) Professor and Head, Department of Computer Science and Engineering, PSG College of Technology, Avinashi Road, Coimbatore-641 004.

(3) Post Graduate Student, Department of Computer Science and Engineering, SNS College of Technology, Coimbatore-641 035.

Table 1: Details of winding Wire Used

Description         MODIFIED             POLYSTERMIDE
                    POLYSTER              POLYAMIDE-
                  PE(MODIFIED)              IMIDE
                                          (PEI+PAI)

Thermal/Insula     55[degrees]         200[degrees] DUAL
tion Class                                   COTED

  IS:              IS: 13730 -13
  IE               IEC 317 - 13

Technica             Fair/Good         Excellent/Excellent
Specifications
Abrasion
Resistance/
Mechanical
Properties

Flexibility And         Good                 Superb
Adherence

Cut-Thro           240[degrees]C   320[degrees]C/220[degrees]C
Heat Shock

Table 2: Details of dielectric test

Dielectric Strength    50 M[OMEGA]   50 M[OMEGA]
before Installation

Dielectric Strength       ZERO       50 M[OMEGA]
after endurance test
run with VFD

HV Test before         2kV--Passed   2kV--Passed
Installation

HV Test after             Failed     2kV--Passed
endurance test
run with VFD